text: Contour advection is a Lagrangian method 
  of simulating the evolution of one or more contours or isolines of
  a tracer as it is stirred by a moving fluid.
  Consider a blob of dye injected into a river or stream: to first order it could be modelled by tracking only the motion of its outlines. 
  It is an excellent method for studying chaotic mixing:
  even when advected by smooth or finitely-resolved velocity fields, 
  through a continuous process of stretching and folding,
  these contours often develop into intricate fractals. 
  The tracer is typically passive as in 
  but may also be active as in, representing a dynamical property of the fluid such as vorticity.
  At present, advection of contours is limited to two dimensions,
  but generalizations to three dimensions are possible.